A conventional environmental control system utilizes various electrically powered devices such as heating, cooling, ventilating, or air conditioning units. Typically, the environmental control system uses a bi-metal thermostat to activate and deactivate the various devices. The conventional bi-metal thermostat directly switches the alternating current which powers the equipment. Since the bi-metal thermostat is a mechanical device, it does not require any external operating power in order to switch the current.
With the advances in semiconductor components and embedded microprocessors, it has become increasingly common to replace the conventional bi-metal thermostat with a sophisticated electronic thermostat which affords a longer life, quieter operation, and greater flexibility. When replacing a mechanical thermostat with an electronic thermostat, additional power must be supplied for circuit operation. Typically, the additional power is provided by an external transformer and an additional power supply line from the transformer to the electronic thermostat. Alternatively, the electronic thermostat may be powered by an internal battery.
There are several disadvantages to providing power to an electronic thermostat by an external transformer. For example, the additional components increase the cost of replacing a mechanical thermostat with an electronic thermostat. In addition, running the conductor from the transformer to the electronic thermostat may be very difficult and costly.
There are also disadvantages to using a battery for providing the operating power. One primary disadvantage is the need to continually check and replace the battery. If the battery is not properly replaced and cannot provide adequate power, the electronic thermostat may fail during a period of extreme environmental conditions.
It would be desirable to provide electronic thermostat switching circuitry with the necessary power for operation by taking, or "stealing" current from the current supply that is being controlled, so that the need for additional wiring or batteries is eliminated.
While it has been shown that FETs have been connected in opposition with common sources for switching alternating current, an overall best scheme for providing the gate control voltages from power derived off the two load control wires for both the OFF and ON states of the control has not been done. Typically, the two gates of the opposing transistors have also been connected together for ON-OFF control without the capability of stealing some of the current for energizing the electronic control circuitry.